The invention relates to a porous ceramic material having piezoelectric properties, such as is used for ultrasonic applications. The invention relates also to a method for making this material.
For ultrasonic antennas or ultrasonic transducers, e.g. for a linear or a phased array used in medical technology, the ideal piezoelectric material fulfills the following four conditions (a) to (d) simultaneously:
(a) A high transducer effectiveness, i.e. a piezoelectric material with highest possible thickness coupling factor K.sub.t ;
(b) Precise emission of short pulses without edge radiation to prevent artifacts due to transverse modes, i.e. a piezoelectric material with a minimum transverse coupling K.sub.p or K.sub.31 and high internal attenuation (i.e. low vibrational Q);
(c) A good electric match between the ultrasonic antenna and the signal line, i.e. the permittivity of the piezoelectric material should be easy to select as a function of the transducer size and frequency; and
(d) a good acoustic match to the object to be examined, i.e. in the case of medical technology a good adaptation to (human) tissue, which means that the acoustic impedance of the piezoelectric material should match the acoustic impedance of the object to be examined as closely as possible.
With conventional piezoelectric materials (piezo-materials) the mentioned requirements (a) to (d) can be fulfilled only to a limited extent, and usually at great expense. Thus, as a rule, the piezoelectric material is selected so that conditions (a) and (c) are fulfilled well. Satisfactory results with respect to requirement (b) can be obtained when the individual transducer elements of the ultrasonic antenna are finely divided and that a damping element is applied on the back of the ultrasonic antenna. To fulfill condition (d), an adaptation layer of .lambda./4 thickness has been provided between the ultrasonic antenna and the human tissue. Much better results can be obtained with two adaptation layers of .lambda./4 thickness, .lambda. being the wavelength of the ultrasound.
By using specific piezoceramics such as PbNb.sub.2 O.sub.6 or low-Q-ceramics (e.g. Vibrit 668), one can fulfill requirement (b) quite well, but allowances must be made for requirements (a) and (c). Adaptation layers are necessary also in this case.
With the use of a piezoelectric plastic (PVF.sub.2) the requirements (b) and (d) are fulfilled very well, but the requirements (a) and (c) only moderately so.
Objects of the invention therefore are to provide a porous, piezoelectric ceramic material which fulfills the above requirements (a) to (d) equally well, with relatively low manufacturing costs. The material should be particularly usable in medical ultrasonic technology, but also in other ultrasonic technologies, as for example materials testing.